Porous gas diffusion elements are well known to those skilled in the art. Thus, for example, they are disclosed in U.S. Pat. Nos. 4,261,932, and 4,262,933; the disclosure of each of these patents is hereby incorporated by reference into this specification.
These prior art porous gas diffusers are often used to diffuse a gas (such as oxygen, ozone, air, carbon dioxide, or nitrogen) into a liquid (such as waste water or drinking water). An ASCE Standard Test ("Measurement of Oxygen Transfer in Clean Water," July, 1984, ISBN 0-87262-430-7) was developed by the American Society of Civil Engineers to evaluate the efficiencies of these prior art devices.
A substantial amount of energy is required to force gas through such porous gels diffusers. The efficiencies of the prior art gas diffusion devices ranged from about 10 to about 40 percent. Thus, even a relatively small increase in such efficiency will result in a significant decrease in energy consumption.
An improved porous gas diffuser is disclosed in applicants' U.S. Pat. No. 5,215,686, which discusses a rigid, monolithic, porous gas diffusion element which contains a ceramic substrate and coating on the top and bottom surfaces of the substrate. As is disclosed in column 6 of this patent (at lines 12-15), the minimum active pore size of the substrate is preferably from about 2 to about 5 times as great as the minimum active pore size of the coating on the top of the substrate.
Although the diffusion element of U.S. Pat. No. 5,215,686 is superior to prior art diffusion elements, it still suffers from certain disadvantages. In the first place, a relatively non-uniform pattern of bubbles is dispensed from its top surface when gas is passed through its plenum; because of this non-uniform bubble distribution, the size of such diffusion element generally must be smaller than about 12 inches. In the second place, the diffusion element of such patent produces relatively coarse bubbles which thus have a relatively short dwell time in the liquid into which they are being dispensed and provide a relatively small surface area for interaction with such liquid. In the third place, because of the relatively short dwell time produced by the prior art device, a relatively long column of liquid must be used to obtain a reasonable gas transfer efficiency, thereby requiring a relatively complicated, costly system.
It is an object of this invention to provide a porous gas diffusion element which produces a substantially more uniform bubble pattern than the diffusion element of U.S. Pat. No. 5,215,686.
It is another object of this invention to provide a porous gas diffusion element which produces a gaseous discharge with a substantially greater dwell time and gas transfer efficiency than that produced by the diffusion element of U.S. Pat. No. 5,215,686.
It is yet another object of this invention to provide a porous gas diffusion element which is suitable for use with a system designed for relatively low flow rates per unit of diffuser surface area.
It is yet another object of this invention to provide a porous gas diffusion element which can be of a size up to about 20 inches and still operate efficiently.
It is yet another object of this invention to provide a porous gas diffusion element which can have a non-circular shape and still operate efficiently.
It is yet another object of this invention to provide a porous gas diffuser with an increased gas transfer efficiency.